The present invention relates to an optical line terminal and an optical line transmission and reception system and more particularly to an optical line terminal and an optical line transmission and reception system adopting a burst-mode optical receiver unit for optimization of a direct current bias of an amplifier.
In the passive optical network (PON), a plurality of optical network units (ONU) are connected to an optical line terminal (OLT) through tree-type topology links and share up bandwidth in the time-multiplexed manner. The optical line terminal utilizes the fixed bandwidth allocation or the dynamic bandwidth allocation (DBA) to allot time slots to the respective optical network units. The optical network unit transmits burst data packets to the optical line terminal in the time slot allotted thereto and an optical receiver unit of the optical line terminal receives the burst data packets transmitted from different optical network unit.
Since the magnitude of power of the burst data packet transmitted from the different optical network unit to the optical line terminal is different, the optical receiver unit of the optical line terminal adjusts a judgment threshold of a limiting amplifier on the basis of the power to perform limitation amplification when the optical receiver unit receives the burst data packet, so that a digital signal is judged exactly. Alternatively, the optical receiver unit of the optical line terminal adjusts a direct current bias for an input signal of the limiting amplifier to attain the same effect as the adjustment of the judgment threshold of the limiting amplifier, so that the judgment of the digital signal can be made exactly. However, a continuous model amplifier in the prior art generally uses AC coupling at an input terminal and signals are distorted during the charging and discharging process of a coupling capacitor therefor, so that relatively long hardware setting time must be used to adjust the judgment threshold of the amplifier. However, in the burst model, such adjustment must be made each time one burst data packet is received and accordingly the utilization factor of the bandwidth is reduced.
On the other hand, the limiting amplifier using AC coupling in the prior art usually employs an automatic bias feedback control circuit to stabilize the judgment threshold of the amplifier to a zero voltage level. Particularly, the cause such as saturation in the leading edge of photoelectric conversion due to excessive input signal power often generated in the burst model subjects the signal transmitted to the limiting amplifier to waveform distortion of different kind. The optimum reception judgment threshold of the distorted signal is larger than the zero voltage level, for example, but is stabilized to the judgment threshold of the zero voltage level by the automatic bias feedback control circuit as usual, so that the receiver unit deviates from the optimum reception of the digital signal and the sensitivity of the receiver unit is reduced.
US 2007/0264031A1 discloses that an MAC control unit of the optical line terminal utilizes the bandwidth allocation information to predict the power of next burst data packet and control the direct current bias of signal on the basis of the predicted power and uses direct current component and reversed direct current signal by charging and discharging of capacitor to cancel influence of the charging and discharging of capacitor, so that signal distortion is removed and the hardware setting time of the amplifier is shortened to improve the system efficiency. In this method, however, the prior-art limiting amplifier using AC coupling is used as usual and the judgment threshold thereof is decided by the automatic bias feedback control circuit, so that the direct current bias of signal is decided by the coupling capacitor (fixed to zero). Accordingly, the optimum reception cannot be maintained by making adjustment on the basis of signal waveform situation.
Furthermore, in this method, a parameter table for generating the direct current bias control signal is a fixed retrieval table. Accordingly, when the system is changed by the cause such as, for example, aging of element and variation in environmental temperature, parameters in the parameter table cannot be varied in accordance with the change, so that the burst receiver unit deviates from the optimum state and the sensitivity of the receiver unit is reduced.
Moreover, how to predict the signal power of an unregistered optical network unit at the time that the optical line terminal begins to transmit signals upon initialization of the system is not described. However, when the optical line terminal uses the relatively short hardware setting time, a registration request of such an unregistered optical network unit cannot be received successfully and it is difficult to register the optical network unit upon initialization of the system or upon recovery after interruption.
U.S. Pat. No. 6,715,113 discloses that a control signal based on an error code indication signal outputted from a forward error correction decoder is used to adjust the judgment threshold of the amplifier and the respective numbers of error codes of 0/1 and 1/0 are balanced to maintain the judgment threshold to be optimum and improve the sensitivity of the system. However, the forward error correction decoder and relatively long time to output the error code indication signal are required (for example, 20 μs) and accordingly it is very difficult for this method to complete feedback control within relatively short time, for example, within one tenth time (for example, 1 to 10 μs) from start of one burst data packet in the passive optical network.